Multiple loop excitation system for plasma



MULTIPLE LOOP EXCITATION SYSTEM FOR PLASMA Filed Nov. 15, 1967 Oct. 2 0, 1970 c, s. ZAROWIN 3 Sheets-Sheet l SIGNAL SOURCE SIGNAL SOURCE gm I 14A- LFIG.1

FIG. 2

lNVIiN'I'UR. CHARLES B. ZAROWIN 13)" BR,

ATTORNEY C. B. ZAROWIN MULTIPLE LOOP EXCITATION SYSTEM FOR PLASMA Filed Nov. 15, 1967 3 Sheets-Sheet 2 SIGNAL SOURCE FIG.4

Oct. 20, 1970 5, z owm I 3,535,653

MULTIPLE LOOP EXCITATION SYSTEM FOR PLASMA Filed Nov. 15; 1967 3 Sheets-Sheet :5

A FIG. 5

E I 0 VOLTS TIME F l G. 6

V 0 I VOLTS TIME F I G 7 POWER o United States Patent 3,535,653 MULTIPLE LOOP EXCITATION SYSTEM FOR PLASMA Charles B. Zarowiu, University Heights, N.Y., assiguor to International Business Machines Corporation, Ar-

monk, N.Y., a corporation of New York Filed Nov. 15, 1967, Ser. No. 683,240 Int. Cl. H01s 3/22; H05h 1/02 US. Cl. 331-945 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to the excitation of gases into the plasma state. The disclosure describes a system wherein gas is contained within two or more topologically toroidal tubes, the tubes having a single common portion or leg. The gas within the tubes is excited into the plasma state by inductive excitation. A driving signal is applied to the primary windings of a transformer whose secondary windings are formed by the plasma within the tubes. The disclosure includes two embodiments. 'In the first embodiment, the transformer primary winding turns are connected in series and in the second embodiment, the transformer primary winding turns are connected in parallel. Each embodiment has particular advantages which will be described.

Cross-references to related applications US. patent application Ser. No. 633,964, entitled Inductive Excitation System for Plasma, filed April 26, 1967, and assigned to the present assignee, is related to the present invention in that a single loop plasma tube is provided having low frequency inductive excitation.

Background of the invention The present invention is in the field of plasma physics and more particularly relates to devices for exciting gas ions into the plasma state. The invention also relates to gas lasers in that the plasma may serve as the active medium for a gas laser.

Description of the prior art The aforesaid application, Ser. No. 633,964, is prior art in that it relates to an inductively excited single loop plasma system. It is distinct in that the features and advantages derived from a multiple loop system cannot be realized in such single loop system. The article Ring Discharge Excitation of Gas Ion Lasers by W. E. Bell, Applied Physics Letters 7, October 1965, relates to a single loop plasma tube inductively excited by high frequency RF signal.

Summary of the invention The present invention is the use of a multiple loop plasma tube having a common portion or leg. The tube is inductively excited by a transformer primary winding connected to a driving signal which may be a low frequency square wave signal. The secondary winding of the transformer is formed by the plasma within the multiple leg tube. The invention is useful in of itself, as a device for exciting gas ions into the plasma state and has further significance in that it can be used as the active portion of a gas laser device generally referred to in the art as a ring discharge laser. The present invention has advantages both in construction and in operation. In the prior art single loop devices it is necessary that one portion or leg of the loop be much smaller in cross-sectional area than the remaining portion of the loop. In the present invention, multiple loops allow the remaining portions of the loops to have a smaller cross-sectional area for the same minimum current density in the narrow portion. This provides a saving in bulk and also makes for simpler Patented Oct. 20, 1970 physical connections since it is difiicult to fabricate a connection from a narrow cross-section to a wide crosssection.

From an operational standpoint, the invention has the advantage that the primary winding turns of the transformer providing the inductive excitation can be con nected in either serial or parallel arrangement. If the primary winding turns are connected in parallel, the input impedance is a function of the product square of the turns ratio of the primary and secondary windings and the load resistance (n R If the primary winding turns are connected in series, the input impedance is a function of the product of the square of the number of legs, the square of the turns ratio, and the load resistance (l n R For the primary winding in series connection, the input impedance is a function of the square of the number of multiple legs employed. For a two loop device, the input impedance in series is four times that of the input impedance for the parallel primary winding connection. For a three-loop device the input impedance in series primary winding connection is nine times that of the input impedance for the parallel primary winding connection. Thus, for a high impedance signal source, the series primary winding connection is employed and for a low impedance generator the parallel primary winding connection would be used.

Still another advantage of the invention is that it permits increased coupling which results in high frequency response.

A further advantage is that the impedance of the plasma is reduced by the use of multiple bypass legs thereby resulting in more power in the active region in the narrow leg.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

Brief description of the drawings In the drawings:

FIG. 1 is an illustration of a multiple loop plasma excitation device having three plasma tube loops and a transformer having the primary windings connected in series.

FIG. 2 is a schematic circuit diagram of the device of FIG. 1.

FIG. 3 is an embodiment of a multiple loop plasma excitation device having three plasma tube loops and a transformer having the primary windings connected in parallel.

FIG. 4 is a schematic circuit diagram of the embodiment of FIG. 3.

FIGS. 5, 6 and 7 are illustrations of waveforms useful in explaining a particular embodiment of the present invention,

Description of the preferred embodiment Referring to FIG. 1, a plasma tube is shown consisting of three loops, each of the loops having one portion in common. The portion in common is the narrow tube 10. The remaining portions of the loops will be referred to as the bypass legs and are designated 12, 14 and 16. The bypass legs 12, 14 and 16 and the narrow tube 10 are hollow and, therefore, each bypass tube and hollow leg 10 form a topologically toroidal surface. The by pass legs 12, 14 and 16 and the narrow tube 10 are characteristically constructed of glass but may consist of other suitable materials. The interior of narrow tube 10 and bypass legs 12, 14 and 16 is filled with a suitable gas such as argon or an argon-xenon mixture at a suitable pressure. Any of the gases heretofore used in ion gas lasers may be employed in the present invention. The narrow leg is surrounded by a core 18. It is noted that core 18 therefore passes through each of the three loops formed by narrow leg 10 and the three bypass legs 12, 14 and 16. A primary winding surrounds core 18 and is shown for purposes of explanation as having six turns. Primary winding 20 is connected to a signal source 22 which provides the driving signal for the excitation. The plasma tube formed by narrow leg 10 and bypass legs 12, 14 and 16 in combination with the core 18 and primary winding 20, is essentially a transformer with winding 20 being a six turn series connected primary winding and the multiple closed gas loop being a three turn secondary winding. Source 22 provides a signal which may be an alternating current signal to produce a changing magnetic field which passes through the loops formed by narrow leg 10 and each of the bypass legs 12, 14 and 16. The changing magnetic fields in turn produces a changing voltage in the gas contained in the tubes to excite the gas into the plasma state. More particularly, the changing voltage in the gas causes the electrons in the gas to move in one direction for positive portions of the voltage cycle and in the opposite direction for negative portions of the voltage cycle. When the electrons move in either direction, they collide with the ions in the gas and give up to the ions the energy supplied by the magnetic fields.

Referring to FIG. 2, an electrical schematic drawing is shown representative of the circuit of FIG. 1. The three series connected primary turns 20A, 20B and 20C are indicated as two turn windings in series to form a six turn series connected primary winding. Turns 20A, 20B and 20C are representative of the six turns of coil 20 in FIG. 1. The three single turn secondary windings 12A, 14A and 16A connected in parallel are representative of the secondary windings formed by the plasma in the loops formed by the bypass legs 12, 14 and 16 in FIG. 1. Resistance R is representative of the resistance of the plasma in the tube of FIG. 1. Resistance R is a negative dynamic resistance being almost infinite prior to the ionization of the gas and falls to a low value in the order of one ohm after ionization depending on the size and geometry of the tube and the pressure of the gas.

Referring to FIG. 3, the plasma tube is identical to that of FIG. 1 consisting of narrow leg 10 connected to bypass legs 12, 14 and 16. In FIG. 3, the six turns of primary Winding 20 are connected such that there are three groups consisting of two turns connected in series. The three two turn groups of windings are connected to each other in parallel. For sake of clarity of the drawing, the core 18 is represented as three separate cores 18-1, 18-2 and 18-3, located about the bypass legs 12, 14 and 16. It is to be understood that this is the equivalent of having the parallel connected primary windings connected on the three cores 18-1, 18-2 and 18-3, all disposed about narrow leg 10. The representation of the primary windings in FIG. 3 was adopted to more clearly indicate the parallel connections. The operation of the device of FIG. 3 is essentially the same as that of FIG. 1 in that the signal from source 22 is used, by means of core 18 and primary Winding 20', to create a changing magnetic field to excite the gas in narrow tube 10 and bypass arms 12, 14 and 16 into the plasma state.

FIGS. 1 and 2 therefore relate to the embodiment wherein the six turns of primary winding 20 are connected in series, and the three turns of the secondary winding formed by the bypass legs 12, 14 and 16 are connected in parallel. Defining the driving voltage from source 22 as E, the voltage across each primary winding is V =E/ l or V =E/ 3. The voltage across the secondary winding is V =V /n or V =E/n1 (Equation 1).

Defining the current in the primary winding as I the current in one leg of the secondary winding is nI In the circuit of FIG. 2, which is representative of the structure of FIG. 1, the secondary winding current through the resistance R is lnI or SnI (Equation 2).

However, V /I =R Comparing Equations 1 and 2, it can be seen that V /I =(E/n1)-(l/n1I which reduces to (V /l =E/n 1 I =)R Since E/I is the input impedance Zin, the expression for the input impedance is Zin=n 1 R FIGS. 3 and 4 relate to the embodiment wherein the six turns of primary winding 20 are connected as three groups of two winding turns connected in parallel, and the three turns of the secondary winding formed by the bypass legs 12, 14- and 16 are also connected in parallel. With the driving voltage from source 22 defined as E, the voltage across each parallel connected primary winding 20A, 20B and 20C is V =E. Since V V n, then VSIE/H.

The current in each of the primary windings 20A, 20B and 20C is I 1. By the usual transformer rules, the current in each leg of the secondary winding is nI 1, however, in the resistance R the secondary currents add to give I :nI The resistance R is equal to Since Zin=E/I then R =Zin/n and Zinzn R It has been shown that in the series primary-parallel secondary embodiment of FIGS. 1 and 2 the input impedance is a function of the square of the number of bypass legs whereas in the parallel primary-parallel secondary embodiment of FIGS. 3 and 4 the input impedanc is not a function of the number of bypass legs.

The use of a plurality of bypass legs as shown in FIGS. 1 and 3 is also advantageous since increased inductive coupling is produced. Since some fraction of the magnetic flux produced by the primary winding leaks out of the loop formed by the secondary winding, increasing the number of secondary winding turns (i.e., bypass legs) reduces the amount of leakage flux. The parallel connected secondary winding as shown in the embodiments of the present invention is desirable because an increase in the number of parallel bypass legs will increase the coupling and thus the high frequency response of the transformer arrangement.

A plurality of bypass legs will increase the coupling between the primary winding and the secondary winding (i.e., the bypass legs) because the leakage flux is the flux from the primary winding which does not go through the secondary winding, and since the secondary winding consists of more turns (i.e., multiple bypass legs), the amount of flux not passing through the secondary turns decreases.

A further advantage of the present invention as shown in the embodiments of FIGS. 1 and 3 is that the overall impedance of the plasma is reduced when a plurality of bypass legs are used, thereby resulting in more power in the active region of the device and less loss in the inactive region. The total impedance of the plasma consists of the plasma impedance of the active region in the narrow tube 10 and the impedance of the bypass legs 12, 14 and 16. The active region in tube 10 has a high current density due to which optical gain necessary for lasing action is obtained. The impedance of a single bypass leg depends on the cross-sectional area. The larger the crosssectional area the lower will be the impedance. A plurality of bypass legs is equivalent to a large cross-sectional area, and therefore, a lower impedance. Thus, more power is dissipated in the active region.

The square wave generator 22 shown in FIGS. 1, 2, 3 and 4 may be an alternating current generator which produces an alternating voltage to provide the changing magnetic field in core 18. Signal source 22 may alternatively be a square wave generator of the type described in the previously mentioned copending application, Ser. No. 633,964. The square wave generator described therein produces a square wave signal as represented by the waveform in FIG. 5. The signal as represented by the waveform in FIG. 5 is applied to the primary winding 20. A square wave secondary voltage V is produced as represented by the Waveform shown in FIG. 6. The voltage V in FIG. 6 is shown smaller in amplitude to the voltage E of FIG. because it is equal to E/nl as set forth in previous Equation 1. The secondary voltage V causes the electrons in the gas within the plasma tube to move in one direction for positive portions of the secondary voltage cycle and in the opposite direction for negative portions of the voltage cycle. When the electrons move in either direction, they collide with the ions and give up to the ions the energy supplied by the magnetic field. Energy is a sealer quantity, thus there is no polarity involved in whether the ions collide with electrons moving in one direction or the other. An illustration of the waveform of the power supplied to the ions is shown in FIG. 7. From FIG. 7 it can be seen that the excitation appears to the plasma substantially as direct current. This is advantageous in that the modulation of the excitation is reduced. Although the excitation appears to the plasma substantially as direct current, the excitation voltage as shown in FIG. 5 being a square wave provides the necessary polarity changes to produce the magnetic field energy which maintains the gas in the plasma state.

In the present invention, the structures of FIGS. 1 and 3 can be considered a gas ion laser amplifier because optical energy is produced in the active region of narrow tube 10. The laser energy may be coupled out of narrow tube 10 by means of conventional Brewster windows 10A and 10B connected to either end of narrow tube 10'. If a laser oscillator is desired, a pair of windows may be added opposite the Brewster windows 10A and 10B.

What has been described is a system for exciting a gas into the plasma state wherein a plasma tube containing the gas is provided having a hollow common tube and a plurality of hollow bypass tubes. The gas is inductively excited by means of a core surrounding one of the loops of the plasma tube and energized by a signal from a primary winding. The primary winding may be connected in series circuit or in parallel circuit and the loops of the plasma tube function as parallel connected secondary windings. It would also be possible to connect the loops of the plasma tube such that they function as series connected secondary windings. In such an arrangement, the plasma tube configuration would be a topologically toroidal loop arranged in a spiral geometry.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A system for exciting a gas into the plasma state comprising:

a plasma tube containing a gas, said plasma tube including a hollow common tube and at least two hollow bypass tubes connected to said common tube to form at least two interconnected topologically toroidal closed loop tubes; and

means coupled to said gas filled plasma tube for exciting said gas into the plasma state.

2. A system according to claim 1 wherein the crosssectional area of said hollow common tube is smaller than the cross-sectional area of said at least two bypass tubes.

3. A system according to claim 1 wherein said hollow common tube is a cylindrical tube sealed at both ends and wherein one end of each of said at least two bypa'ss tubes are physically connected to said common tube at an opening in said common tube proximate to one end of said common tube and wherein the other end of each of said at least two bypass tubes are physically connected to said com mon tube at an opening in said common tube proximate to the other end of said common tube.

4. A system according to claim 1 wherein said means coupled to said gas filled plasma tube includes coupling means for imparting energy to said gas and signal generating means connected to said coupling means for producing said energy.

5. A system according to claim 4 wherein said coupling means includes at least one core passing through at least one of said closed loop tubes and a primary winding wound on said at least one core and wherein said signal generating means is connected to said primary winding for producing a changing magnetic field around said at least one core for exciting said gas in said plasma tube into the plasma state.

6. A system according to claim 5 wherein said primary winding consists of a plurality of winding turns connected in series circuit.

7. A system according to claim 5 wherein said primary winding consists of a plurality of winding turns connected in parallel circuit.

8. A system according to claim 5 wherein said coupling means includes a magnetic core encircling said common tube and a primary winding having a plurality of turns wound in series circuit on said magnetic core.

9. A system according to claim 5 wherein said coupling means includes a plurality of magnetic cores encircling at least one of said closed loop tubes and a primary winding wound on each of said plurality of cores and connected in parallel circuit.

10. An ion gas laser comprising a plasma tube containing a gas, said plasma tube including a hollow common tube and at least two hollow bypass tubes connected to said common tube to form at least two interconnected topologically toroidal closed loop tubes and means coupled to said gas filled plasma tube for exciting said gas to cause a population inversion in said gas to produce continuous wave operation of said ion gas laser.

Goldsborough et 211., RP Induction Excitation of CW Visible Laser Transitions in Ionized Gases.

RONALD L. WIBERT, Primary Examiner R. J. WEBSTER, Assistant Examiner US. Cl. X.R. 

